The physiological and subjective changes produced by inhaled doses of nicotine will be studied in frequent and infrequent tobacco smokers. Tolerance development and abstinence phenomena will be evaluated. In two successive, day-long laboratory sessions, smokers will smoke tobbaco cigarettes containing between zero and four milligrams of nicotine every half hour for eight hours. Their blood nicotine level will be monitored and their brain wave activity, cardiovascular functioning and subjective reactions will be recorded. Tolerance development will be followed and inferred from diminished response magnitudes and from reproduction of initial effects following the smoking of a higher nicotine dose. The time course and intensity of incotine abstinence phenomena will be studied while subjects smoke placebo cigarettes. The final abstinence symptoms will be evaluated. Detailed smoking and drug use histories, health histories and self-reports of reasons for smoking and personality traits will be obtained prior to the subjects' participation and will be used in conjunction with the laboratory data to develop selection criteria whereby appropriate treatments can be matched with the particular individual qualities of a smoker wishing to stop smoking.